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³Cat-2

A 6-Unit CubeSat flying a novel GNSS-R payload for Earth observation.

The 3Cat-2 (spelled “cube-cat-two”) is the second satellite in the 3Cat series and the second satellite developed in Catalonia at UPC’s NanoSat Lab. This spacecraft carries the PYCARO main payload, a novel dual-band and dual-polarization GNSS Reflectometer (GNSS-R) which has been designed and manufactured at UPC’s Remote Sensing Lab and NanoSat Lab. It also integrates the Mirabilis Star Tracker, an experimental star tracker for attitude determination and the AMR eLISA magnetometer designed and manufactured at IEEC for the future ESA’s LISA mission.

Spacecraft class

6-Unit CubeSat

Total mass

7.1 kg

Dimensions

100 mm x 243,7 mm x 340.5mm

Mission status

Launched

Phase E: Operations

Launch date

August 15th, 2016

On-Board Computer and Flight Software

The 3Cat-2's On-Board Computer is based on an ARM7 microcontroller running at 40 MHz and using up to 2MB of RAM. The system runs FreeRTOS, an open source real time operative system based on a very simple kernel. The OBC is integrated in a commercial available board which also includes a low power 3-axis magnetometer, which is used for the attitude determination purposes, and a microSD slot fitted with a 2 GB card containing the satellite's file system, which is based in FAT16.

On top of the FreeRTOS runs the flight software which is charge of the satellite control including: sensors reading, communication protocol stack, attitude determination and control algorithms, payload managing and command and data handling. Being both a very simple microcontroller and software, the system is robust and predictible, reducing the mission risks.

Electric Power Subsystem

The 3Cat-2's Electrical Power System is in charge of harvesting energy via solar panels to then manage and store it in batteries. The 3Cat-2 has a total of 5 surfaces covered with Triple Junction Gallium Arsenide solar cells from AzurSpace with an effiency of 30% in BOL. To store the energy the satellite has a 7.4 V Lithium-Ion battery pack with a total capacity of 7000 mAh.

Communication Subsystem

The communication system is based on available comercial systems for CubeSats and is split in a total of three diferent links integrating the amateur protocol AX.25. A UHF link working at 1200 bps and modulated in AFSK (Bell 202 standard) is in charge of the uplink while the downlik relais on a VHF link transmitting at 9600 bps and an S-Band link transmitting at 115200 bps, both modulated in BPSK.

The UHF and VHF links are working together as a full-duplex system and controlled by a Selective Repeat protocol, which is able to reliable upload and download files. UHF-VHF system is in charge of telemetry and command upload, and for that reason two UHF-VHF transceivers are placed in the satellite, as redundancy of the system. On the other hand, the S-Band downlink is a “blind” link, which gives the user the possibility of downlink a file without any uplink command. For that reason, an erasure code is used on top of the file sent. In this case a LDPC-Staircase is used for the mission, which encapsulates the scientific payload data generated and sends towards the Earth at pre-defined times.

In addition, the communications subsystem also includes a beacon in the VHF band which periodically transmits a set of telemetry data in an open format and can be easily received from Earth following these simple steps.

Attitude Determination and Control Subsystem

The attitude determination and Control System is responsible for orientating and stabilising the spacecraft towards a desired target attitude. In order to achieve this, a set of sensors and actuators are used to know the current orientation of the spacecraft, perform slew maneuvers, and counteract orbital disturbances.

On the one hand, the 3Cat-2 employs a set of three orthogonal magnetic torquers (0.24 Am2 each) as actuators. This type of actuation relies on controlling the spacecraft dynamics by generating a torque owing to the interaction between the Earth’s magnetic field and a local dipole moment from the magnetic torquers. On the other hand, the sensors selected for 3Cat-2 are two 3-axis magnetometers (fine and coarse) to measure the Earth’s magnetic field, one 3-axis MEMS gyroscope to measure angular rates, and a set of six photodiodes mounted on each face of the nanosatellite to determine the Sun’s position. Additionally, an in-house, experimental star tracker is to be tested on orbit (not included in on-board determination algorithms) to assess its performance in space.

³Cat-2 CAD model, exploded view

Payloads

3Cat-2 is carrying up to 4 payloads, the main payload, PYCARO, plus the Mirabilis star tracker, the eLISA magnetometer developed by IEEC and finally a compression algorithm for the scientific data downlink developed by DAPCOM, the FAPEC Compression Algorithm.

PYCARO GNSS-R main payload: PYCARO from P(Y) and C/A ReflectOmeter, is the main payload of 3Cat-2, based on the technology GNSS-R. GNSS-R is based on collecting the opportunity signals sent by the GPS, Galileo, Glonass and BeiDou constellations to perform reflectometry. The final goal of the scientific data of a GNSS-R mission is to perform altitude maps of the Earth, wind surface over the sea, deforestation... The payload has dual polarization, RHCP and LHCP, working in both L1 and L2 bands of GPS but also works for any of the actual GNSS constellations.

Mirabilis Star Tracker: The Mirabilis is an experimental star tracker developed by the NanoSat Lab. This star tracker, which is not included on the attitude determination algorithms, flies in the 3Cat-2 mission in order to be validated for future Nanosat Lab missions such as 3Cat-3.

IEEC AMR eLisa magnetometer: This magnetometer, developed by the Insitut d'Estudis Espacials de Catalunya (IEEC) will be included in the future ESA's LISA mission and for this reason, it's behavior will be analysed and validated during the 3Cat-2 mission.

FAPEC compression algorithm by DAPCOM: The FAPEC is a compressions algorith by DAPCOM which is in charge of compressing the scientific data generated by the 3Cat-2 payloads. It is the first time this algorithm is used in a space mission

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